A 14,000-Mr envelope protein of vaccinia virus is involved in cell fusion and forms covalently linked trimers.

A monoclonal antibody, MAbC3, that reacts with a 14,000-molecular-weight envelope protein (14K protein) of vaccinia virus completely inhibited virus-induced cell fusion during infection. Immunoblot and immunofluorescence studies revealed that the 14K protein was synthesized at about 6 to 7 h postinfection and transported from the cytoplasm to the cell surface. Synthesis and transport of the 14K protein during infection occurred in the presence of rifampin, an inhibitor of virus maturation. One- and two-dimensional gel electrophoretic analyses demonstrated that the 14K protein forms largely trimers (42K) that are covalently linked by disulfide bonds. The facts that MAbC3 prevents virus uncoating and blocks virus-induced cell fusion but does not prevent virus attachment to cells and the 14K envelope protein forms trimers all suggest that this protein plays major role in virus penetration.